The process of transferring motion picture video material to video tape or disc involves a change in the rate at which the video frames are displayed from the respective media. In the U.S., film is conventionally projected at a rate of twenty-four frames per second, while video tape or disc is standardized to run at a nominal rate of thirty frames per second. To minimize flicker, each video frame is scanned twice, at double the frame rate, to provide two half-frames called fields. One field contains odd-numbered scanned lines; the other field contains even-numbered scanned lines. This technique is referred to as interlaced scanning.
The difference in frame rate between the film and video is addressed by a process illustrated in FIG. 1 that takes advantage of the interlaced video scanning technique. Since the video and film frame rates have a 5/4 ratio, every four film frames 10, 12, 14, 16 must be recomposed to form five video frames 18, 20, 22, 24, 26, each consisting of two fields, or a total of ten fields from the four film frames 10-16. To meet this requirement, alternate film frames are scanned to form three video fields (film frames 10 and 14 ) or two video fields (film frames 12 and 16). This process, referred to as 3:2 pulldown, yields three video frames 18, 24 and 26 composed of two fields from a single film frame, and two video frames 20, 22 composed of fields from adjacent film frames, for every sequence of four consecutive film frames and five corresponding video frames. When the video medium is viewed at normal speed, the fact that the frames 20, 22 are composed of fields from adjacent film frames does not present a problem. However, if either frame 20 or 22 is viewed in a freeze-frame mode, the video anomalies caused by obtaining the interlaced fields from adjacent film frames can cause annoying jitter on the viewing screen. It is therefore desirable to provide a mechanism for enabling viewing of video frames 18, 24 and 26 in a freeze-frame mode, while preventing viewing of frames 20 and 22. The present invention is directed to this need.
Conventionally, attempts have been made to identify the video frames suitable for freeze/frame viewing by manually observing the 3:2 pulldown sequence illustrated in FIG. 1, and thereafter coding the video frames by number under the assumption that the sequence illustrated in FIG. 1 is repeated throughout the video medium. However, the sequence illustrated in FIG. 1 can be changed or interrupted due to editing, for example, requiring that the sequence thereafter be manually reestablished. Furthermore, this manual process becomes more tedious and complex in the event that the film is run at a non-standard or variable film speed. For example, very old motion pictures were recorded using cameras cranked by hand, often resulting in significant variation in film speed from the twenty-four frame-per-second standard speed. Indeed, prior to implementation of the present invention, it had been considered all but impossible to transfer these early motion picture materials to video tape or disc while at the same time identifying video frames suitable for freeze-frame viewing as outlined above.
It is therefore a general object of the present invention to provide a system, apparatus and method for identifying frames of video information consisting of substantially identical fields suitable for freeze-frame viewing that operate automatically without operator intervention, that automatically accommodate variations in film speed, and/or that enable freeze-frame viewing of video frames consisting of fields that are sufficiently similar to each other as to reduce or eliminate the annoying jitter that would otherwise result. Another and related object of the present invention is to provide a system and method for transferring frames of video information from one medium such as motion picture film to another medium such as video tape or disc having a differing frame rate that automatically identify the video frames on the second medium consisting of fields drawn from the same frame or substantially similar frames of the first medium, termed "clean" video frames hereinafter, and that distinguish such frames from video frames on the second medium consisting of fields drawn from adjacent frames on the first medium that are not substantially similar. Another and related object of the present invention is to provide a system and method for identifying such frames on the second medium in such a way as to enable freeze-frame playback of the second medium only of frames composed of fields drawn from the same frame or sufficiently similar frames or fields of the first medium.